Capacitors of the type typically used in motor run applications have a housing and a capacitive element of wound polymer film within the housing. The housing is at least partially filled with an insulating fluid. The capacitive element is connected to terminals on the exterior of the housing, and a pressure responsive interrupter system is associated with the terminals and conductors connecting the capacitive element with the terminals. If the capacitive element fails, it may do so in a sudden and violent manner, producing heat and out-gassing such that high internal pressures are developed within the housing. The pressure responsive interrupter system is designed to break the connection between the capacitive element and the terminals in response to the high internal pressure, thereby removing the capacitive element from a circuit and stopping the high heat and over-pressure condition within the housing before the housing ruptures.
The housings have often been constructed of metal, with the terminals and pressure responsive interrupter system on a metal cover of the metal housing. It had always been assumed that metal enclosures were necessary for strength, due to the high internal pressures that are developed within the housings during a fault or run-away defective condition, and due to the force required to lift or bend the metal cover to insure disconnection of the capacitor terminals from the internal capacitive element. However, in recent years capacitors with plastic housings have appeared on the market, including pressure responsive interrupter systems that operate by bending or lifting a plastic cover. Typical capacitors with plastic enclosures are shown in U.S. Pat. Nos. 5,019,934, 5,148,347 and U.S. Pat. No. 5,381,301.
In order to insure that the pressure responsive interrupter systems of plastic housing capacitors perform consistently and reliably, it was found necessary to underfill the housings with insulating fluid, leaving a substantial amount of air within the capacitor housing. The additional air is believed to act as a cushion during the violent, uncontrolled reaction that takes place when sudden failure of the capacitor is induced by the rigorous standards of UL testing, and similar reaction can take place when the capacitor fails under real-world situations. When the capacitor housing is entirely or nearly filled with insulating fluid, the plastic housing tends to rupture, but when the housing is partially filled with insulating fluid, the plastic housing is able to maintain integrity while the pressure responsive interrupter system operates in its intended manner.
However, one of the major drawbacks of leaving air space in this type of capacitor is that the capacitive element may not be totally immersed in the insulating fluid. This is especially true if the capacitor is mounted in a horizontal position or in an inverted position, and inverted mountings are becoming increasingly popular. When the insulating fluid does not fully cover the capacitor element, at least a portion the end spray area of the capacitive element is exposed to the air and possible development of corona, leading to capacitor degradation. Additionally, the capacitive element tends to generate heat during use, and the heat is best dissipated through the surrounding insulating fluid. When areas of the capacitive element are not covered by fluid, they develop excessive heat which has a deleterious effect on long term capacitor performance. Typically, there is a reduction in capacitance value to below acceptable limits, an increase in dissipation factor which indicates internal arcing, and eventual separation of end spray material. This excessive capacitive loss and increase in dissipation factor will usually result in a runaway failure mode, with the sudden and violent reaction requiring successful operation of the pressure responsive interrupter system.
Nevertheless, a capacitor with a plastic housing has certain advantages over a capacitor with a metal housing. These advantages include lower costs, and the insulating properties of a plastic housing compared to the conductive properties of a metal housing. Overcoming the drawbacks of the prior capacitors with plastic housings would therefore be an advance in the art.